Solid mixing processes are usually described with the help of the dispersion model. Dispersion is based on stochastic particle movements which are responsible for the concentration balance between the different components. For simplification, homogeneity adjustment of the mixture is mostly described by the axial mixing direction. In this research, the one-dimensional dispersion model is extended to three dimensions. A modified sampling strategy shows how the dispersion model can be applied to the mixing process in axial, radial, and tangential directions. Small differences between axial and radial mixing efficiencies are observed for the 45 degrees-configuration of the mixing tool, whereas a higher mixing efficiency is detected in tangential direction. The 90 degrees-configuration behaves differently. Anisotropic dispersion coefficients are determined for the different mixing tool configurations. These indicate that particle mobility in axial direction is lowest. (C) 2014 Elsevier B.V. All rights reserved.